How Lava Created Strange Spirals on Mars

New images from the HiRISE camera aboard the Mars Reconnaissance Orbiter reveal curious spiral shapes on the surface of Mars. How did they get there?

Andy Ryan, a planetary geologist at Arizona State University, didn't notice these enigmatic coils on the Martian surface at first. It wasn't until he took a second, closer look at images of the Red Planet that the odd spirals came into focus.

In a study published today, Ryan described the shapes as lava coils, spiral formations that show up on the surface of a lava flow and endure when it solidifies. His discovery marks the first time that lava coils have been documented on another planet—with most of these measuring 5 to 10 meters across, they were too small for previous Mars observations to detect. And these peculiar shapes give scientists new insight into the geologic history of our planetary neighbor.

Ryan was looking into an area called the Athabasca Valles, a large (about 100,000 square miles), canyon-like valley near Mars' equator. At the head of the valley is a series of cracks called the Cerberus Fossae, and at the base the valley opens into a huge, flat area. For the past few years scientists have debated whether a volcanic eruption or the influence of ice near the surface created the polygonal landscape features within the area. "I became really enamored by the ice-versus-lava debate," he says.

There are several huge polygonal shapes in the area that bear a distinct resemblance to ice floes. But Ryan says most of the evidence favors fire over ice. First, the formation is found near the Martian equator, where ice is rare. And these coils couldn't have been formed by freezing water, he says, because water solidifies too slowly to form spirals.

Lava, on the other hand, does form coils; they appear in relatively recent lava flows on Earth, especially in areas such as Hawaii. "Their formation is a fairly basic process," Ryan says. "Whenever you have a lava flow, you can have shear zones where parts of the flow are moving past each other at different speeds or even in different directions." The movement of those different parts can cause the lava to rotate. It then cools as it spins to create a permanent coil on the surface of the flow.

The direction of the coil can tell planetary geologists how the lava was moving as it flowed, and reveals that the surface of the vast area is coated with lava. More than 5000 cubic kilometers worth of lava pooled in the Cerberus Palus on Mars, forming a giant lava lake. "It's one of the largest lava flows in the solar system," NASA Goddard Space Flight Center planetary geologist Chris Hamilton says.

Laszlo Keszthelyi, a University of Arizona geologist who reviewed the paper, says, "On Earth, we would call it a sea." Keszthelyi says there have been eruptions of a similar size here on Earth in the distant geologic past, but none recently.

NASA

Ryan spotted these coils with the HiRISE (High Resolution Imaging Science Experiment) instrument, a camera on board the Mars Reconnaissance Orbiter that takes detailed images of Mars' surface, and which planetary geologists use to study the planet's history and composition. Alfred McEwen is the principal investigator for the HiRISE instrument, which has taken 24,128 pictures of Mars in the past five years, mapping about 1 percent of the surface in fine detail. "What I find most interesting about it [the study] is that these things are hiding in the HiRISE data," he says.

Hamilton agrees, saying that HiRISE could uncover plenty more weird formations on Mars. "These things that seemed featureless are still intriguing . . . and we have a lot more to look at."

And if Ryan is right that it was lava, not ice, that left these graceful spirals drawn on the surface of Mars, other questions remain. For one thing, scientists still aren't sure how the Athabasca Valles itself formed. The eruption that caused the lava coating and its coils might not have been enough to carve the entire valley, and researchers speculate that an outpouring of water from the same area at an earlier time could have been responsible. Others say it's possible for a massive flood of lava to have carved the channel, but without more evidence it's hard to know for sure. "What came out of the underworld is a mystery," Hamilton says.

Either way, Ryan says, the coils tell us something interesting about our neighbor planet. Their excellent preservation, along with the lack of craters in the cooled lava, suggests the volcanic eruption didn't take place all that long ago. "The most popular consensus lies around the 20-million-year mark," Ryan says.

Twenty million years—a long time for humans but an instant in geologic time. "The age of this unit really implies that Mars is not just geologically active but volcanically active, and we can expect to see this again in the geologically recent future," Hamilton says.

But don't start a Martian volcano watch just yet. It will probably be millions of years before the Red Planet has another eruption.

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